In the drilling of wells by the rotary method, a drilling fluid is used which generally consists of an aqueous clay suspension often containing weighting agents to increase the hydrostatic head and frequently also containing concentrated colloidal suspending and conditioning agents.
The drilling fluid serves to bring cuttings to the surface, to cool the bit and to keep the oil, gas and water confined to their respective formations during the drilling process. For these functions, it is necessary that the drilling fluid be of pumpable viscosity, have sufficient carrying capacity to bring cuttings to the surface, and yet be fluid enough to release cuttings and entrained gas at the surface.
A highly important property of drilling muds is the ability to form an impervious filter cake upon the permeable walls of the bore hole, thus inhibiting further ingress of water from the drilling fluid into the formation. Excessive fluid loss from drilling fluids can cause severe problems. For example, filter cake build-up can become so thick that the drill pipe may become stuck. Also, there may be great difficulty in withdrawing pipe from the hole. Also, high water losses can cause sloughing and caving in of shale formations. In addition, electrical logging of the well can be adversely influenced due to the mud filtrates, etc.
Various water loss control agents have been previously proposed to improve the properties of drilling fluids, but not all of these have been successful.
Applicants have discovered tetrapolymers which have attractive properties as fluid loss control agents.
Accordingly, an object of this invention is to provide an improved drilling fluid.
Another object of this invention is to provide an improved method of drilling a well.
Another object of the invention is to control the rheological properties of drilling fluids.
Other objects, aspects, as well as the several advantages of the invention will be apparent to those skilled in the art upon reading the following disclosure and appended claims.